In a vacuum device used in a semiconductor manufacturing process, an etching device, a thin film deposition device, and other process chambers are connected via gates to a transfer chamber with a robot that introduces and brings out semiconductor wafers that are specimens, and load lock chambers for changing the wafers that are specimens are connected via gates to the transfer chamber. Each of the process chambers and the load lock chambers is connected to a vacuum pump via an exhaust channel with an opening and closing valve, so that both process chambers and load lock chambers as well as the transfer chamber are evacuated.
Considering, for instance, the load lock chambers, there are a variety of operating states such as a period of performing evacuation from the state of being open to the atmosphere for changing specimens, a period of wafer change by the robot, and a period of closure of the load lock chambers. In these periods, exhaust capability is controlled by the opening and closing operations of the opening and closing valves of the exhaust channels.
During the operation of such a vacuum device, if the motor of the vacuum pump is stopped in the middle of manufacturing so that the vacuum of the vacuum device cannot be maintained, products being processed become defective. Therefore, in order to avoid such a situation, the vacuum pump is controlled so as to be driven continuously and constantly. Further, its rotational speed is set so as to be always constant irrespective of the operating state of the vacuum device.
It has been proposed to reduce the number of revolutions of a motor driving a vacuum pump when the power consumption of the motor becomes higher than or equal to a predetermined value in order to protect the vacuum pump from overloading while maintaining the vacuum of a vacuum device (see Japanese Laid-Open Patent Application No. 2000-110735). It has also been proposed to urge maintenance before a vacuum pump comes to a stop by measuring a physical quantity such as case temperature or motor current values that vary during the operation of the pump and issuing an alarm when the measured value reaches a preset value (see Japanese Laid-Open Patent Application No. 5-118289).
The object of these proposals is to avoid a situation where the vacuum device cannot maintain a vacuum by protecting the vacuum pump from overloading. They have no intention to control power consumption in the vacuum pump.
When a vacuum pump is driven at a constant rotational speed, the vacuum pump may rotate at a greater number of revolutions than is required depending on the operating state of a vacuum device. The vacuum pump cannot be stopped during the operation of the vacuum device, and depending on the operating state, it is driven at a greater number of revolutions than is required, thus causing needless power consumption.